It depends entirely on what you are trying to do.
Dolby Digital is supported on every DVD-Video player in both PAL & NTSC, whereas DTS is optional. More people have DD decoders than DTS.
DTS is percieved as higher quality than DD, but to be blunt there really isn't much, if anything, in it.
DTS is brutally expensive. The SurCode DTS-DVD encoder is $2000 USD. Dolby Encoders are cheaper, but a hell of a lot more complex to learn. It is far from a case of "hit the button" as you must understand all the metadata settings, or you will run into problems.
If it is guaranteed compatibility with every DVD player you need, then get DD.
Oh, AFAIK there are only a couple of Authoring apps that can import DTS files. Scenarist, at $30,000 or at the other end of the scale Pegasys TMPGEnc DVD author.
get this from www.pegasys-inc.com
hope this helps.